The electrochemical lithium insertion properties of highly crystalline Li x Mn 2O 4 are investigated in the approximate lithium insertion range 0 < x < 4. Using liquid and PEO based polymer electrolytes at room and elevated temperatures, it is shown that for this material, the reversibility of the lithium insertion/extraction reactions is better at the higher voltages (versus Li/Li +), and particularly at the 4 V plateau. The lithium insertion/extraction reaction at the 1 V plateau although essentially reversible is associated with a significant voltage hysteresis.